1. Field of the Invention
The present invention relates to a dual balanced optical signal receiver for use in a practical coherent lightwave communication system.
2. Description of the Related Art
In a lightwave communication system, a direct detection method is usually employed in which a received optical signal transmitted via an optical transmission line is directly supplied to an optical receiving element and then transformed thereby into an electric signal. In a coherent lightwave communication system, a so-called homodyne detection method or a heterodyne detection method is known, in which a very pure laser device is used as a local oscillation optical source and the local oscillation optical signal is mixed with the received optical signal. This method has a superior receiver sensitivity to that of the aforesaid direct detection method. Accordingly, in the coherent lightwave communication system, it is possible to enlarge the repeater spacing, and thus reduce the number of repeaters. Further, where the coherent lightwave communication system is introduced in a subscriber network, it is possible to increase the number of branches, and thus realize an economical arrangement of the optical transmission lines.
Recently, in such homodyne detection or heterodyne detection techniques, there has arisen a demand for a suppression of a reduction in interference efficiency due to a fluctuation of a polarization plane between the received optical signal and the local oscillation optical signal, and a reduction in receiver sensitivity due to an intensity noise of the local oscillation optical signal (hereinafter referred to as intensity noise).
To cope with the above demand, various techniques have been proposed in the prior art, as will be explained in detail hereinafter. The problems occurring in the prior art optical signal receiver are, first it is difficult to increase receiver sensitivity since a part of both the received optical signal and the local oscillation optical signal is left unused, and second, it is difficult to maintain a suppression of intensity noise contained in a local oscillation optical signal regardless of a fluctuation of the polarization.